The present invention is directed to a method for enhancing the formation and recovery of dialkyldihalosilane from alkylhalosilane crude. More particularly, the present invention relates to a direct method for methylchlorosilane manufacture by maintaining a low weight ratio of zinc with respect to copper while using phosphorus as a catalyst for enhancing dimethyldichlorosilane formation.
The Rochow Direct Method Process for making methylchlorosilanes by effecting reaction between powdered silicon and methyl chloride in the presence of a copper catalyst is shown by U.S. Pat. No. 2,380,995. As taught by Rochow in Chemistry of the Silicones, Second Edition, (1951), John Wiley and Sons, New York, pp. 79-80, dimethyldichlorosilane is the source of dimethylsiloxy, or "Di" units when hydrolyzed, and is basic for the manufacture of high molecular weight polymers. It has been long recognized, as shown by Rochow et al., J. Am. Chem. Soc., 63, 798 (1941) that in addition to copper, key metallic promoters, such as zinc and tin, also can enhance the formation of dimethyldichlorosilane during direct method methylchlorosilane crude production.
T. Margaria et al., WO/95/01303, (1994) have further described the use of phosphorus as a promoter for dimethyldichlorosilane formation in the direct method. Copper phosphide has been cited by Halm et al., in U.S. Pat. No. 4,762,940 (1990), as a useful source of phosphorus for improving dimethyldichlorosilane selectivity in direct method procedures. As shown in U.S. Pat. No. 5,059,343, Halm et al., also investigated the effects of using phosphorus to improve dimethyldichlorosilane selectivity in direct method operations in combination with copper, tin and zinc. Even though significant advances have been made with respect to procedures for improving the yields of dimethyldichlorosilane in direct method methylchlorosilane manufacture, further techniques are constantly being sought.